Is a day trip US to Canada currently practical?

The answer is yes, with a bit of preparation from your end. Below is from personal experience of doing such a day trip.

I think I understand why a plan is required: it's to give border agents more discretion to decide I should not be exempt from quarantine

The official quarantine information page gives the following rationale for the quarantine plan:

Plan your quarantine. Even if you are fully vaccinated, you still need a quarantine plan in case it is determined at the border that you do not meet the necessary requirements.

So your quarantine plan will essentially be... "hop into my car, turn around and go back to New York". Its not there in case you're infected during your stay, its only there in case the border agents want you to quarantine upon arrival. However the online form doesn’t fully support this scenario so enter the name of any hotel next to the border or whatever hotel you’ll be spending the night at. This was confirmed to me by the CBSA agent at the border and by OPs experience in the accepted answer.

I need to get tested 72 hours before entry. I took the Walgreens test Thursday. The pharmacist said to expect the results Monday, and explained that the 72-hour turnaround they offer is "business hours". (How that math works out is beyond me.)

Yes, the processing time for free COVID tests is not guaranteed but usually it should be okay. Your options are:

1. Hope that the free test comes within 72 hours (which it should most of the time). If it doesn’t, take another test next week and try again.
2. Get an express PCR test with a guaranteed turnaround. The closest place to Cornwall that I could find is "Garnet Testing Center" in Burlington, they charge $250 for a same-day result. If you search around, there might be a premium test company closer to where you live.

Additionally be aware that you might be randomly selected for a second COVID test right as you cross the border. Trying to argue with them by saying you’ll be back in the US before the test results come in unfortunately didn’t work from personal experience, so you might be forced to spend around 20 minutes after crossing filling out additional paperwork and getting your test done. That second test is free of charge. In theory you might get a call afterwards saying that you’re positive and asking you to start your quarantine plan but you’ll be long gone by the time it happens so said phone call is going to be moot.

The third way this venture might not work: what's the likelihood US customs will let me back in?

If you're a US citizen or permanent resident, there's nothing for you to worry about. The official rule states the following:

For purposes of the temporary alteration in certain designated ports of entry operations authorized under 19 U.S.C. 1318(b)(1)(C) and (b)(2), travel through the land ports of entry and ferry terminals along the United States-Canada border shall be limited to “essential travel,” which includes, but is not limited to

— U.S. citizens and lawful permanent residents returning to the United States;

You might think that citizens need an "essential" reason too but that's not true - only foreigners need valid reason like work, medical care or diplomatic travel. US citizens/Green Card holders are "essential" by the very fact that they're returning home, no other reason required. As a side note, crossing the US land border is currently amazing and looks exactly like it was in 2019: you roll on to the border guard, show your ID, they say "welcome home" and you're on your way. No tests, no vaccine card to show, no one asks about any symptoms, no pre-registration required - its beautiful.

The land borders are closed to non-essential travelers.

https://www.cbc.ca/news/canada/us-border-canada-closed-1.6147189

"Canada's land border with the United States will remain closed until at least Sept. 21."

https://ca.usembassy.gov/covid-19-information-canada-3/

"Land border restrictions for travel to the United States remain in effect through September 21, 2021 and may be extended. For information on travel restrictions for Canada, see Entry and Exit Requirements below."

Remember that if you are a USA citizen and you actually are in Canada, you are allowed to go back to the USA, even by car.